1. Field of the Invention
The present invention is related to a liquid crystal display device, particularly to a liquid crystal display device disposing a pair of insulating substrates opposite to one another with a predetermined gap by interposing spacers therebetween, holding liquid crystal compounds, and having an additional capacitance portion formed in a pixel area, a manufacturing method of the liquid crystal display device, and a fabrication apparatus for the liquid crystal display device.
2. Description of the Related Art
High-resolution liquid crystal display devices capable of color display for notebook-sized computers and computer monitors are spread widely.
Those liquid crystal display devices are generally classified into a passive matrix type in which what is called a liquid crystal panel is formed by interposing a liquid crystal compound layer (hereinafter also referred to simply as “liquid crystal”) between two insulating substrates at least one of which is a transparent glass plate or the like and pixels are formed by applying voltages selectively to various pixel forming electrodes formed on the insulating substrates of the liquid crystal panel and thereby changing the alignment direction of liquid crystal molecules in each desired pixel portion, and an active matrix type in which various electrodes as mentioned above and active elements for pixel selection are formed and pixels are formed by changing the alignment direction of liquid crystal molecules in each desired pixel portion by selecting part of the active elements.
In general, active matrix liquid crystal display devices employ what is called a vertical electric field scheme (also called a TN (twisted nematic) scheme) in which electric fields for changing the alignment direction of the liquid crystal are applied between an electrode formed on one substrate and electrodes formed on the other substrate.
On the other hand, liquid crystal display devices employing what is called a lateral electric field scheme (also called an IPS (in-plane switching) scheme) have been put in practical use in which the directions of electric fields applied to the liquid crystal are approximately parallel with the substrate surfaces. For example, a liquid crystal display device of the lateral electric field scheme is known in which comb-tooth electrodes etc. for electric field formation are formed on one of the two insulating substrates, whereby a very wide viewing angle is obtained.
In a liquid crystal display device of the lateral electric field scheme, a plurality of scanning signal lines (hereinafter referred to as “gate lines”) and video signal lines (hereinafter referred to as “drain lines”), switching elements formed in the vicinity of crossing points of the gate lines and the drain lines, pixel electrodes to which drive voltages are applied via the respective switching elements, and counter voltage signal lines (hereinafter referred to as counter electrodes) are formed in an active matrix substrate (also called “thin-film transistor substrate”). Color filter layers are formed in a color filter substrate so as to correspond to respective pixels that are formed in aperture regions of a black matrix that is made of a resin composition. A liquid crystal panel is formed by interposing a liquid crystal between the active matrix substrate and the color filter substrate. The liquid crystal display device is constructed by disposing a backlight behind the liquid crystal panel and integrating them using top and bottom cases.
Image display is performed by varying the light transmittance of the liquid crystal by means of electric field components that are formed between the pixel electrodes and the counter electrodes and are approximately parallel with the surfaces of the insulating substrates.
In contrast to the case of the vertical electric field scheme, such a liquid crystal display device of the lateral electric field scheme provides a clear image even when viewed from a large viewing angle with respect to the display surface (a large inclination from a normal to the display surface). Hence, the liquid crystal display device of the lateral electric field scheme hence is superior in so-called viewing angle characteristic.
For example, Japanese Unexamined Patent Publication No. Hei. 6-160878 and its counterpart U.S. Pat. No. 5,598,285 discloses a liquid crystal display device having such a configuration.
FIG. 17 is a plan view showing one pixel, an associated light shield region of a black matrix, and its peripheral region of an exemplary liquid crystal display device of the lateral electric field scheme. Each pixel is formed in a region enclosed by four signal lines, that is, a gate line GL, a counter voltage signal line CL, and two adjacent drain lines DL.
Each pixel includes a thin-film transistor TFT as a switching element, a storage capacitance portion Cadd, a pixel electrode PX, and counter electrodes CT. A plurality of gate lines GL and a plurality of counter voltage signal lines CL extend in the right-left direction and are arranged in the top-bottom direction in FIG. 17. A plurality of drain lines DL extend in the top-bottom direction and are arranged in the right-left direction in FIG. 17. Each pixel electrode PX is connected to the associated thin-film transistors TFT and each counter voltage signal line CL is integral the associated counter electrodes CT. Each thin-film transistor TFT is formed by the associated gate line GL as a gate electrode, a drain electrode that is formed on a semiconductor layer AS formed on the gate line GL and extends from the associated drain line DL, and a source electrode that is connected to the associated pixel electrode PX.
Each pixel electrode PX is opposed to the associated counter electrodes CT. Display is controlled by modulating transmission light or reflection light by controlling the alignment state of the liquid crystal by electric fields developing between each pixel electrode PX and the associated counter electrodes CT. The pixel electrodes PX and the counter electrodes CT are long and narrow electrodes extending in the top-bottom direction in FIG. 17 and assume comb-tooth shapes.
The potential of each counter electrode CT is stable because it is supplied externally via the associated counter voltage signal line CL. Therefore, almost no potential variation occurs in each counter electrode CT though it is adjacent to a drain line DL. Further, because of the presence of counter electrodes CT, each pixel electrode PX is geometrically distant from the adjacent drain lines DL, whereby the parasitic capacitance between those electrodes is decreased to a large extent. This makes it possible to control a variation of the pixel electrode potential due to video signal voltages.
As a result, crosstalk (i.e., an image quality defect called “vertical smear”) in the top-bottom direction can be suppressed.
A specific example is as follows. The widths of the pixel electrodes PX and the counter electrodes CT are set at 6 μm, which is sufficiently greater than 4.5 μm, which itself is greater than a maximum setting thickness of the liquid crystal (described later). It is desirable that the widths of the pixel electrodes PX and the counter electrodes CT be set sufficiently greater than 5.4 μm because it is preferable to secure a margin of 20% or more in consideration of processing variations in manufacture.
With the above measure, electric field components applied to the liquid crystal that are parallel with the substrate surfaces become larger than electric field components in the vertical direction, which makes it possible to prevent undue increase of voltages for driving the liquid crystal. It is preferable that the maximum values of the widths of the various kinds of electrodes be smaller than the interval L between each pixel electrode PX and the counter electrodes CT associated therewith.
This is for the following reason. If the electrode interval L is too small, electric field lines are curved very much and electric field components that are perpendicular to the surfaces of the insulating substrates are larger than those parallel with the substrate surfaces. Therefore, the electric field components parallel with the substrate surfaces cannot be applied efficiently to the liquid crystal.
To prevent disconnection, the width of the drain lines DL is set a little greater than the widths of the pixel electrodes PX and the counter electrodes CT. To prevent short-circuiting between each counter electrode CT and the drain line DL adjacent to it, the interval between them is set at about 1 μm. Further, the drain lines DL and the counter electrodes CT are formed in different layers in such a manner that the drain lines DL are formed above a gate insulating film that covers the gate lines GL and the counter electrodes CT are formed below the gate insulating film.
On the other hand, for the following reason, the interval L between each pixel electrode PX and the associated counter electrodes CT is changed in accordance with the liquid crystal material used. The electric field strength that attains maximum transmittance depends on the liquid crystal material. Therefore, the electrode interval L is set in accordance with the liquid crystal material so that maximum transmittance can be obtained in a range defined by a maximum amplitude of a signal voltage that is set in accordance with a breakdown voltage of a video signal driving circuit (signal-side driver) used. With a liquid crystal material described later, the electrode interval L is set at about 15 μm.
In the liquid crystal display device being considered, the black matrix BM formed on the color filter substrate (not shown) seems to be located above the gate lines GL, the counter voltage signal lines CL, the thin-film transistors TFT, and the drain lines DL, and an aperture periphery of the black matrix BM formed seems to reveal that gaps between the drain line DL and the counter electrodes CT in a plan view.
Outside each aperture periphery of the black matrix BM (i.e., outside each pixel region), each additional capacitor Cadd is formed by the associated pixel electrode PX and counter voltage signal line CL and an insulating film formed in between.